Thinking a long time on this but keep backing off. I have the F4 IS and it's wonderful. Am I missing something really special in not trying a lens most working pros consider a given?

Do you "need" the more shallow DOF of f/2.8 over f/4? Do you "need" the lower noise of, say, ISO 1600 over ISO 3200? Do you "need" whatever resolution increase the f/2.8L II IS offers over the f/4L IS? Do you "need" AF with a 2x TC (unless you have a camera that will AF at f/8, where the f/4L IS will also AF with a 2x TC)?